FAMILY CABDIACEA. 225 



The shell is stout, covered with a dense black fibrous epidermis, 

 and it is usually tinged internally with rose or salmon colour. 

 The hinge is more rudely developed than in the other two species. 

 The front teeth are blunt and irregular, crested with notches, and 

 they are supported by a thicker collateral deposit of shelly matter, 

 reaching to the cicatrix of the anterior adductor muscle. Poste- 

 riorly the elongated tooth is reduced, as in Anodonia, to a blunt 

 marginal ridge ; and to compensate for this the external ligament 

 is longer and stronger. 



Family II. CARDIACEA. 



Animal bearing a thin horny shell, with the mantle lobes open anteriorly for 

 the passage of a large protruded foot, and united posteriorly to form the 

 branchial and excretory siphons ivhich are prolonged into tubes wholly or 

 partially united. 



The dykes of Holland and western Germany, and the fen? of 

 East Anglia, elicited, forty and thirty years ago, the attention of 

 two able observers to their little bivalve mollusks, in Carl Pfeiffer, 

 of Cassel, and the Eev. Leonard Jenyns, of Cambridge. Before 

 their researches, the little horny cockles of our rivers, canals, and 

 stagnant waters were associated together in one genus. Their mantle 

 lobes are open anteriorly for the passage of a capacious extensible 

 foot, while they are united posteriorly, as well as ventrally, to form 

 the branchial and excretory siphons, which are prolonged into 

 tubes. Both siphons and foot are largely extensible, and the 

 animal has a marvellous power of withdrawing and turning them 

 about, and often performing a whirling rotary motion by forced 

 ejectments of the water. But of some, the siphons are united in 

 one tube, in others, they are united for a little distance and then 

 separated. These two forms of Cardiacea constitute the genera 

 JPisidium and Cyclas; and not only are there differences in the 

 animal, but also in the shell. In Pisidkim, the anterior side of the 

 shell is the longer ; in Cyclas, the anterior side is the shorter. 



The freshwater cockles are chiefly inhabitants of the temperate 

 regions of the globe in both hemispheres. About an equal number 

 of both kinds are known, from five-and-twenty to thirty species of 

 each, but in the Eastern Hemisphere the number of Pisidia com- 



Q 



